1. Field of the Invention
The invention relates to a method and a system for producing a power supply voltage for controlling an electronic switch, for example a triac, more particularly for producing a direct current power supply voltage for supplying power to electronic control circuits when the switch is connected to two wires and controls the supply of power to a load (for example a lamp), i.e. in particular when the switch is connected in series with the load between live and neutral in a single-phase alternating current mains power supply.
2. Description of the Prior Art
In the context indicated above, the electronic switch (the triac) replaces a conventional switch with metal contacts. It is therefore placed with its electronic control circuits in a housing built into the wall, at which only two wires usually terminate, referred to hereinafter for convenience as the xe2x80x9clivexe2x80x9d wire and the xe2x80x9cswitched livexe2x80x9d wire. The electronic switch is of course connected between the available two wires, i.e. between the live wire and the switched live wire. The load is connected between the switched live wire and the neutral wire, which is not accessible.
A direct current voltage of a few volts, for example 5 volts, is required to power the electronic control circuits of the switch. With the wiring configuration indicated above, this direct current voltage cannot be obtained from the two wires available in the housing built into the wall.
However, the document EP 0 031 626 describes a direct current power supply circuit for controlling a triac which takes the necessary electrical energy from the mains supply via a capacitor and a rectifier circuit. The rectifier circuit can produce a usable direct current voltage only if the triac is turned off. Consequently, the solution proposed in the above prior art document entails interrupting conduction of the triac in each cycle for the short time interval that is needed to produce a direct current voltage that can be used to supply power to the control circuits of the triac.
This kind of solution, which entails periodically creating a xe2x80x9cconduction stopxe2x80x9d in the control cycle of the triac, generates interference that requires the use of a costly and bulky filter if the system is to conform to the applicable standards.
The invention proposes a different solution, which relies on a residual voltage at the terminals of the switch while it is conducting.
To be more precise, the invention provides a method of producing an auxiliary power supply voltage for controlling an electronic switch connected in series with a load to a single-phase alternating current mains supply, which method uses a residual voltage drop between the terminals so that the electrical energy needed to produce the auxiliary power supply voltage while the electronic switch is conducting is taken from the terminals of the electronic switch.
In practice, the alternating current residual voltage drop is of the order of one volt. It is therefore advantageous to rectify and step up the residual voltage, for example using a prior art rectifier/voltage step-up circuit. In this way a voltage of the order of 5 volts or more can be obtained, which is perfectly suitable for the direct current power supply of the control circuits of the electronic switch.
If a capacitive component, or even a small electrochemical storage cell, is connected to the output of the rectifier/voltage step-up system, it is possible to maintain a usable voltage when the electronic switch is not conducting (open), and it is therefore possible to control the power circuits of the electronic switch normally when the triac begins to conduct again, i.e. to supply power to the load. However, while the electronic switch is turned off, it is advantageous to produce the auxiliary power supply voltage directly from the mains voltage; this is because, when the electronic switch is turned off (open), the auxiliary power supply voltage can be produced directly from the alternating current mains supply using a simple rectifier circuit connected to the live wire by a capacitor, for example.
The invention also provides an electronic system for controlling an electrical power supply for a load, the system including an electronic switch adapted to be connected in series with the load to supply it with power from a single-phase alternating current mains supply, control means for the switch, and a first auxiliary direct current power supply circuit connected to supply power to the control means, wherein the first auxiliary direct current power supply circuit has an alternating current electrical energy input and the control means are adapted to connect the alternating current electrical energy input to the electronic switch when it is conducting.
As previously mentioned, the first auxiliary power supply circuit preferably includes a rectifier/voltage step-up system, for example a regulated switch-mode power supply.
Integrated circuits specifically designed for rectifier/voltage step-up systems of the indicated type are available off the shelf, for example.
The LINEAR TECHNOLOGY(copyright) LT1073-5 integrated circuit is specifically designed for this application, for example.
The above system preferably includes a second direct current auxiliary power supply circuit connected at the input to the alternating current mains supply and adapted to supply power to the control means when the electronic switch is not conducting.
The second auxiliary power supply circuit consists of a simple rectifier connected to the mains supply via a capacitor.